With the development of the power and electrical techniques, especially, the development of high-power silicon controlled rectifier (SCR) manufacturing, DC power transmission has gain wider and wider applications in electric power systems. A cascaded multi-terminal HVDC power transmission system is composed of three or above converter stations and a DC power transmission line, wherein more than one converter stations operate as a rectifier station or an inverter station. As compared with a two-terminal HVDC power transmission system, in the following situations, for example, a cascaded multi-terminal HVDC power transmission system may operate in a more economical and flexible manner: collecting electric power from multiple electric power bases (for example, wind farms) located in a large area for outward transmission; transmitting a large amount of electricity from an energy base to several remote load centers; providing access to power supplies or loads on mid-branches of a DC line; realizing asynchronous networking of several independent AC systems through a DC line; for the power transmission of metropolis areas or industrial centers, transmitting power energy to several converter stations through DC power transmission, where cables must be used due to limits on overhead power line corridors, or AC power transmission is unsuitable due to limits on short-circuit capacity.
In a cascaded multi-terminal HVDC power transmission system, it is inevitable for high voltage devices, such as converters, smoothing reactors, DC filters, etc. suffering from the impacts of high voltage, large current, the natural environment and connected AC systems to have failures. In the case of faulty part of the system (such as, a converter on a certain stage), it is desired to cut such part off from the system reliably while keeping other parts of the system operating normally, so as to ensure the safety of the HVDC power transmission system and improve its energy availability.